Lithium vs SLA duty life...

I’m trying to compare the total cost of 12V 3AH batteries and while the Lithiums are about five times the cost of the SLA, it is my understanding that they have a much longer service life, but I’m not finding hard numbers to do the math.

Is it possible to make a rule-of-thumb estimate on the total cost of the two types considering service life? Is Li going to be cheaper in the long run?

Hard numbers : Lithium-iron-phosphate is the one you want. 5000 cycles at 80% DoD, 7000 at 50%.

Sealed Lead Acid - roughly 500 cycles to 50%. To 20%, even for so called “deep cycle” batteries damages the battery significantly.

Now, which is cheaper for you depends on 2 other key factors. Just because the battery that is 5 times as expensive last 10-12 times longer doesn’t mean it’s worth it. You need to specify the application, and the expected service cycling, and what your discount rate is. (what are you investments giving you in risk free return, or what is the interest on your mortgage?)

Then you can set up a spreadsheet, factoring in that discount rate by month or year, to see which is worth it. In some applications, you need the additional performance that lithium offers, such as in a sailboat, where there is limited room for batteries, limited capacity for carrying their weight, and you benefit from greater capacity as it gives you a deeper energy buffer and options to run things like reverse osmosis units and refrigeration on solar power, increasing your time at sea. If you were living in an off grid house all year, it might also be worth it.

An RV you only play with occasionally? SLA. Same with a cabin you only spend time in occasionally. You could instead have put the extra money to reduce the balance on your mortgage (or credit card debts), which is a guaranteed return.

I hope the LiPo4 batteries eventually become cheap, because they are far superior in every meaningful metric. They are not what are in a Tesla or phone or laptop, they have lower energy density, but they have very long longevity and have a significantly lower fire risk.

Oh, here’s a complete kit. 24V 10Ah Headway LiFePO4 Battery Kit

$179. This is obviously direct from China, where everything is made, and yes there is some risk. Don’t install something like this if it isn’t in a fireproof box with a vent to the outside. 24v * 10 Ah * 0.8 usable capacity* = 192 watt-hours.

The batteries you are comparing to? They are 12v * 3Ah * 0.5 usable capacity = 18 watt-hours. So $179 + shipping gets you the equivalent of 10 of the lead acid batteries you are comparing to. I suspect lithium may actually be cheaper. Now, it’s 24 volts, and this store doesn’t have any configured for 12, though there are ways to get that. Depending on what you are using it for, this might or might not be a problem. 24 volt input inverters are commonly available. And again, this is a made in China (and engineered in China it looks like at these prices), so it might be of low quality, and it might start a fire. I’d want it in a metal electrical box outside a house, or inside a metal container with a vent to the outside if in an RV. I’d pay more for more expensive batteries if it were a sailboat.

  • by usable capacity, if you go below 20% on a lithium-iron battery or 50% on a deep cycle lead acid, you destroy the battery. You’ll get a fraction of the normal cycles.

Lithium-ion batteries also have a finite life, even if it’s just sitting on a shelf. It’s hard to find good numbers, but if it needs to be used for more than a few years to pay for itself, it may not make it - i.e. it may degrade from aging before it degrades from charge/discharge cycles.

Depends on the type and the level of charge. Lithium iron phosphate, the type I am recommending, lose 1-2% per year. Lithium cobalt - the type used in phones, laptops, and EVs lose a lot more, 5-10%, but it depends on state of charge. If you keep them fully charged they lose 10% or more per year.

I use various sizes of 12v LiFePO4 batteries for emergency communications equipment, but I also have several similar-capacity AGM batteries. The Li batteries are a lot more expensive up front, but they have these advantages:

  • Much lighter
  • Much less self-discharge, so can be stored for months without tending and still retain most of their charge
  • More available capacity. They deliver about 99% of nameplate capacity, vs 50-60% for SLA or AGM batteries (depending on the discharge rate).
  • Much higher charging rate, given a compatible charger. The LiFePO4 batteries I have can be charged within 1 hr, which is 6 amps for a 6 Ah, 20 amps for a 20 Ah, and 50 amps for a 50 Ah battery
  • Better performance in cold temperatures
  • Much greater cycle life, about 3,000 cycles to 80% depth of discharge vs 500 cycles to 80% DOD for the ones I use
  • Can be stored or charged in any position (in general AGM and SLA also can, but check the spec sheet for your specific battery. Sealed gel-type batteries often cannot be).

Always use a compatible charger for the specific battery type, and double-check it adheres to the battery spec sheet for charging rate. In general Li-ion storage batteries are safe, but they hold a lot of energy. You don’t want a thermal runaway or fire from a high-capacity Li-ion battery which was caused by an incompatible charger.

Do not use a non-deep-discharge battery like an automotive starting battery in a deep discharge application. There are lots of lithium-ion motorcycle starting batteries floating around, but these are not designed for deep discharge use. Depending on the vendor you purchase from and the battery brand, it may not be immediately obvious whether the battery is designed for deep discharge use. Always validate this.

If you have multiple 12v DC batteries and devices, I suggest you convert them all to a common connector type called Anderson Powerpole. This ensures all devices and batteries can interoperate without using a proprietary manufacturer-specific connector:

The AGM batteries I use are Tempest: TD Series - Tempest Deep Cycle AGM Batteries

The LiFePO4 batteries I use are Stark:

Lithium based chemistries have flatter outputs where the supplied voltage remains quite near the nominal value for a large portion of the discharge cycle. Whether this is important or not is application specific but it is seldom a disadvantage.

Besides being a company with an interesting site with cool products, Powerstream has a reference document page the might be of use to OP.